1. Field of the Invention
The invention relates to a method for removing sediment-forming materials, such as nonvolatile components ("waxes"), from citrus oils. More particularly, the invention relates to a process for eliminating latent waxes by treating citrus oils with an aqueous acidic treatment solution.
2. Description of Related Art
A well-known problem with cold-pressed citrus oils is that they tend to develop sediments during storage. Formation of these sediments is primarily attributable to the presence of certain nonvolatile components in the oils, commonly and generically referred to as "waxes". The removal of sediments can be a tedious and expensive process. Therefore, the art has searched for ways to reduce the wax content of citrus oils, and thereby minimize sediment formation. Unfortunately, no method known to date has proven to be completely effective.
The most common commercial "dewaxing" procedure involves chilling freshly-produced citrus oil well below its normal storage temperature. By chilling the oil, the solubility of the waxes present in the oil is reduced and some of the waxes tend to precipitate out of solution. The precipitates are then removed by centrifuging the chilled oil. A soluble portion of the waxes, however, remains in the oil after this process.
If the soluble waxes remaining in the citrus oil were not susceptible to any subsequent chemical changes, this commercial dewaxing procedure would provide a shelf-stable citrus oil, because these waxes would remain soluble indefinitely at normal storage temperatures. However, a principal weakness of this procedure is that some of the waxes do undergo chemical changes during storage. For example, psoralen epoxides, which are relatively soluble in cold citrus oils and thus are not removed by chilling-dewaxing, gradually undergo hydrolysis in solution to yield the corresponding diols. These diols are much less soluble than their epoxide precursors in the citrus oil, and they precipitate. In a typical citrus oil kept under typical storage conditions, this transformation occurs slowly but continuously, resulting in increasing amounts of sediment deposition over a period of weeks to years.
Some of the psoralen epoxides not removed by chilling are also known to be phototoxic, making some commercially-available citrus oils unsuitable for use in fragrances and other products applied to the skin.
Attempts have been made to improve upon conventional chilling-dewaxing processes. U.S. Pat. No. 2,863,861 to Platt describes a process for treating citrus oils to remove "wax-crystallization-inhibiting" compounds ("inhibitors") prior to conventional dewaxing by chilling (i.e., chilling, brief cold storage and centrifugation). These unidentified compounds are said to inhibit the crystallization or separation out of waxes in citrus oils during chilling, thereby impairing the effectiveness of wax removal by chilling. In the described process, the citrus oil is washed with a buffered aqueous dispersion of an active esterase or pectase enzyme which is claimed to be capable of destroying the inhibitors, for a period of time deemed sufficient to allow the enzyme to act on and destroy the inhibitors. The enzyme dispersion is buffered to a pH ranging from 4.0 to 7.0 using sodium citrate and citric acid. The washed oil is then separated from the aqueous portion.
The method of U.S. Pat. No. 2,863,861 specifically addresses the problem of wax-crystallization-inhibiting compounds; however, it does not address nor even recognize the separate problem of the conversion of oil-soluble psoralen epoxides to insoluble diols during storage of the citrus oil. The method is not effective in preventing the development of sediments during subsequent storage of the treated citrus oils, because the method does not reduce nor eliminate the amount of psoralen epoxides in the citrus oil. The prior art's lack of effectiveness in this respect has been confirmed by high performance liquid chromatography (HPLC) analysis, which allows for measurement of the epoxide and diol concentrations in citrus oil before and after processing.
Japanese Patent Laid-Open No. 84457/1988 discloses a process for removing phototoxic furocoumarins (psoralens) from lemon oil. In this method, lemon oil is first distilled at reduced pressure to remove any furocoumarins from the distillate. The residue is then dissolved in a solvent and reduced with LiAlH.sub.4. Water is then added to decompose the LiAlH.sub.4, followed by solvent extraction and subsequent distillation to remove the extraction solvent. The residue of the second distillation is then combined with the distillate from the first distillation to yield the final product.
This complex technique is expensive due to the need for costly reagents and energy-intensive separation techniques. In addition, the method can cause undesirable changes in the volatiles content and hence the flavor of the oil due to the distillation process.